Field of the Invention
The invention relates to a simple process for the preparation of totally or partially blocked polyisocyanates and to their use, in particular, in light-stable and weather-stable polyurethane (PUR) coating materials.
As crosslinking agents for the formulation of PUR coating systems, especially PUR power coatings, wide-spread use is made of blocked polyisocyanates, in particular aliphatic and/or (cyclo)aliphatic polyisocyanates, since these lead to light-colored products and, specifically, the cyclo(aliphatic) crosslinking agents can be ground and are therefore particularly suitable for the production of powder coatings.
In PUR powder coatings, which were first described in the literature in 1970, a leading position has been taken by the PUR powders disclosed in DE-C 27 35 497, owing to their superior weathering stability and thermal color stability.
In addition to blocked polyisocyanates which may contain isocyanurate groups, other oligomers are also known, such as polyisocyanates which contain urethane, biuret or urea groups and whose NCO groups are likewise blocked. In order to be able to produce stable two-component coating systems, the blocking of the NCO groups is unavoidable for temporary protection. In this context mixtures are also used.
A common feature of all these blocked polyisocyanates is that they are synthesized batchwise in a reactor, as described in, for example, DE-C 27 12 931. Accordingly, the synthesis of the oligomers mentioned is carried out batchwise. Long reaction times at high temperatures are a disadvantage here, which may lead to discoloration of the reaction products. Furthermore, long reaction times mean high process costs.
Crosslinking agents based on isocyanurates, i.e. trimerized diisocyanates, have received particular interest because of their relatively high molecular weight and their functionality of .gtoreq.3, a prime example being the isocyanurate of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate, IPDI).
The high melt viscosity both of the isocyanurate and, even more so, of the blocked crosslinking agent necessitates high temperatures when the reactants are used directly, as is described in DE-C 27 12 931 and DE-C 33 28 130. Moreover, at these high temperatures which lead to instances of discoloration and cloudiness, there are already high concentrations present of free blocking agent and therefore, correspondingly, high concentrations of free--i.e. unblocked--NCO groups, since the reaction equilibrium is on the side of the starting materials to an undesirably high extent. In addition, the evaporation of the blocking agents causes odor pollution. Since this solvent-free route was fraught with problems, these crosslinking agents have hitherto been synthesized in solvent, at relatively low temperatures of from 80.degree. to 100.degree. C.; in this procedure virtually all of the NCO groups can be reacted.
A disadvantage of syntheses involving such a solvent based procedure is that the solvent has to be removed again subsequently if the blocked polyisocyanates are to be used in the powder coatings field. This measure would also have to be carried out at mild temperatures, below the unblocking temperature, in vacuo with the use of thin-film evaporators, filmtruders or extruders, but hitherto it has not been technically possible to carry out this measure given the extremely high viscosities of these products.
The object of the present invention is therefore to provide a less complex process which is beneficial to product quality, or, in general, a process, for the preparation of totally or partially blocked polyisocyanates having low to extremely high viscosities for the formulation of PUR coating systems, which process does not have the disadvantages mentioned.
Surprisingly it has been found that the blocking of the NCO group and/or chain extension in a direct, continuous, solventless procedure in the melt, despite the high melt viscosity, if certain process conditions are observed; in many cases chain extension can be carried out concurrently with the blocking, in the form of a one-step process. In accordance with the process described below, polyisocyanates can be blocked inter alia at high temperatures, already well within the range in which the blocking groups are detached, without instances of cloudiness and/or discoloration occurring in the reaction product and without greater contents of free NCO being formed than in the synthesis of the crosslinking agent in solvent.